There is an increasing interest in providing articles of manufacture that include one or more support layers formed from a thin film polymer. A “thin film polymer” is understood to be a polymer having a thickness of 1 mm or less. This polymer has found to be a good substrate or carrier layer on which electrically conductive traces can be formed. Also, this polymer, even though thin in cross section, has sufficient mechanical strength that it can also support the mounting of mechanical, electromechanical and electrical components. Another feature of this polymer is that even though it is capable of supporting components and conductors, it is flexible. Thus, this polymer can serve as a substrate for an assembly that, owing to its intended use, may require components that are disposed on a non-linear surface.
One such article of manufacture is an implantable medical device. These devices are implanted into a living being, human or species, to perform for diagnostic and or therapeutic reasons. One such device is an electrode array. This type of device includes some sort of carrier or frame on which plural exposed electrodes are mounted. Conductors, also part of the array, function as the array components over which currents are sourced to or sunk from the individual electrodes. Some electrode arrays are further constructed so that the actual components from which current is sourced to or sunk from the array are also mounted to the array. The array itself is designed for implantation against the tissue of a living being, including a human. More particularly, the array is positioned so that electrodes are able to flow current through tissue so that current flow will result in the desired physiological effect on the patient. Selective current flow through a patient is used for or has been proposed for the following therapeutic reasons: correcting cardiac arrhythmia; pain management; appetite suppression; control of incontinence; and the overriding of damaged neurological connections that have resulted in loss of muscle control and/or loss of feeling. Still another application of these arrays is to monitor the electrical impulses generated by the individual's neurological system. The electrodes of the array transmit signals representative of these electrical impulses to components off the array. The off array components may are able to use these signals to control the devices to which they are connected. These devices include, but are not limited to, mechanically powered exoskeleton units that move the individual, robotic linkages and artificial speech generators.
The Applicant's Assignee's FOLDABLE, IMPLANTABLE ELECTRODE ARRAY ASSEMBLY AND TOOL FOR IMPLANTING SAME, PCT Pub. WO 2009/11942 A2, U.S. patent application Ser. No. 12/873,397, US Pat. Pub. No. US 2011/0077660 A1, its IMPLANTABLE ELECTRODE ARRAY ASSEMBLY INCLUDING A CARRIER FOR SUPPORTING THE ELECTRODES AND CONTROL MODULES FOR REGULATING OPERATION OF THE ELECTRODES EMBEDDED IN THE CARRIER AND METHOD OF MAKING SAME, PCT Pub. No. WO 2011/017426 A2, U.S. Pat. Pub. No. US 2012/0310316 A1, the contents of which are incorporated herein by reference, disclose versions of these electrode arrays. Generally the electrode arrays of these publications include a frame, sometimes called a carrier, formed from an elastic material. Electrodes are disposed over these frames. The frames of these disclosures are formed from Nitinol, a nickel titanium alloy. Given the conductive nature of these frames, it is necessary to form the electrodes themselves over electrically insulating layers. These documents state that it may be desirable to apply parylene-C to the elastic Nitinol carrier so that this material, once cured, functions as the insulating support layer. These documents actually state that it may be desirable to apply plural layers of parylene. Each layer, once cured, functions as the layer upon which one or more conductive components are formed. For example, the cured parylene layers closest to the elastic carrier serve as support layers on which conductors are formed. The outer layers of the parylene serve two functions. First these layers serve as the electrically insulating skin of the array. Secondly, at least one of these parylene layers typically also functions as the support layer over which the array electrodes are formed.
Parylene is a good electrical insulator, bonds well to superelastic material like Nitinol, is flexible once cured and accepts metal layers that are selectively etched to form conductors and electrodes. These are desirable qualities for an insulating layer that is part of an implantable electrode array. However, parylene has been found to have a characteristic that limits its suitable as an insulating layer for an implantable electrode array. Specifically, parylene absorbs relatively high quantities of water. An electrode array implanted into living tissue is surrounded by body fluids. These fluids are primarily water. Given the parylene tends to absorb water, there is a concern that, over time, a significant quantity of these bodily fluids could be absorbed into the parylene layers of the electrode array. This fluid, once absorbed into the parylene, can force the insulating layer to delaminate from the layers to which it is bonded. This delamination of the insulating layer can, in turn, result in the breakage and subsequent malfunctioning of the array itself.
Accordingly, there is an increasing interest in forming the insulating layers out of polymer other than parylene. One alternative polymer that can be employed in an electrode array as an electrically insulating layer is a liquid crystal polymer. This polymer, like parylene, has good bonding properties, is flexible when bonded, and accepts metal layers. In comparison to parylene, a liquid crystal polymer absorbs appreciably less water. Once implanted in a living being, the LCP insulating layer or layers of an electrode array absorb nominal amounts of body liquid and, by extension, are less prone to delaminate.
The Assignee's incorporated by reference PCT Pub. No. WO 2011/017426 A2, discloses that an electrode array with LCP insulating layers can be formed by first mounting some components to the array frame. Then the polymer, in the liquid state, is applied to the partially assembled array and allowed to cure. This method of assembly has been found to be expensive. Accordingly, there is an interest in forming implantable electrode arrays with liquid crystal polymer layers wherein the LCP itself is already in sheet form.
However, to date, it has proven difficult to manufacture electrode arrays with LCP that is already in the form of a cured sheet. This is because the sheets when applied to the frame or other layer over which it is bonded often seats unevenly over the underlying surface. This makes it difficult, if not impossible, to then apply the metal layers on the LCP insulation layer in a manner that ensures that conductive layers and/or electrodes remain bonded to the insulating layer.